1. Field of the Invention
The present invention relates to a method to binarize a multivalued halftone image, and in particular to a method for binarization when digital multivalued color halftone image as ready by scanner is reproduced by a color output device, which can output binary values.
2. Technical Background
In the field of image processing, ordered dither method or error diffusion method are generally known as the technique to binarize a multivalued color image for tone reproduction.
Ordered dither method is a binarization technique, by which one pixel of input signal as read from an original of multivalued image corresponds to one pixel of the binarized image and the input signal is compared with the threshold table having periodicity fixedly corresponding to each position of an input pixel to determine whether "to output" or "not to output".
The error diffusion method is a binarization technique, by which the error in binarizing one pixel of multivalued image is dispersed and added to the surrounding input pixels according to the size of the coefficient of the relatively fixed coefficient matrix at the site where error occurred.
On the other hand, a method is known in the field of printing, by which color film is used as input medium and is separated into colors, periodic dots are prepared on black-and-white films by optical method using contact screen, printing blocks are prepared using these films, and color image is reproduced as dot image. In case of dot image, the size of dot represents the density of the image. Further, a method is known, by which the contact screen is rotated at different angle for each color, and moire and color difference due to the registering miss on each color is reduced.
Also, another method has been developed to prepare dots electronically without using optical screen, and this has been furnished in the scanner for printing or plotter system in recent years.